Printed circuit boards are well known and have numerous applications. Printed circuits are used in almost all types of electronic equipment ranging from radio and television sets, telephone systems units, automobile electronic wiring, guided-missile and airborne electronic equipment, computers, and industrial control equipment. The printed circuit board has evolved from one-sided printed circuit boards to two-sided through-connected printed circuit boards, to multilayer circuitry, and to flexible circuitry.
Flexible printed circuits are useful in applications where flexing is a functional requirement, such as where flexing facilitates assembly and disassembly, or other dimensional variations need to be accommodated. Conventional flexible circuit materials include the flexible plastic films KAPTON, NOMEX, and TEFLON (trademarks of Du Pont). These materials provide flexibility, stability, and heat resistance, and are bondable to copper sheets for the formation of a printed circuit board, and to themselves to provide an insulating layer for the printed circuit and/or to facilitate the construction of multilayer boards.
Multilayer printed circuit boards increase the available surface area in an available space for the mounting of circuits and circuit parts. Conventional multilayer circuit boards comprise of a number of boards having etched copper foil circuits disposed thereon, separated by an insulating layer, e.g., prepreg, laminated or compression molded together under controlled temperature, pressure, and time. The circuits defined by the various conductor layers are connected by through holes and connection holes which are drilled and plated by traditional methods.
Rigid-flexible circuit boards are generally composed of separate rigid and flexible layers which are bonded together. The separate layers are either rigid or flexible insulating substrates which can be provided on one or both sides with a copper laminate (circuit). The shape of the rigid layers defines the rigid area of the printed circuit board. The flexible layers are, therefore, defined by the shape of the rigid layer. The flexible regions of the printed circuit board join the rigid regions of the circuit together or provide flexible connections to external assemblies. Electrical connection between the copper layers is provided by plated through holes
German Patent DE 36 24 718 describes a rigid-flexible printed circuit board which has flexible regions constructed with flexible, adhesive coated plastic layers. These layers are free of peripheral gaps. When the individual layers are manufactured, a tool is used to punch or cut notches in preimpregnated boards (prepregs). These notches are somewhat larger than the desired flexible region of the printed circuit board. Plastic layers, which have been similarly prepared, are inserted into the prepregs. The thickness of the prepregs is the same as the thickness of the plastic layer. Copper foil is applied on the top and bottom of a preliminary laminate to form a composite. This composite is compression molded. Printed circuit patterns are subsequently etched in the copper foils.
U.S. Pat. No. 4,715,928 describes a method for manufacturing single layer rigid-flexible printed circuit boards. These printed circuit boards include one layer of printed circuit. The use of flexible polyimide films, possibly precoated with glue, to provide the desired flexible region in the finished printed circuit board is disclosed. The flexible film may be bonded to the rigid region through the use of prepregs.
A rigid-flexible printed circuit board which has defined regions of mechanically laid wire conductors is disclosed by U.S. Pat. No. 4,533,787. This printed circuit board is provided with slots which define flexible regions. The slots may be cut so as to provide for crosspieces in the slots. The slots are introduced before application of a flexible film. Ultimately, the region surrounded by the slots is removed.
U.S. Pat. No. 4,533,787 also discloses that a composite film, which does not act as an insulating film, may be situated in the slots. The composite comprises a thermoplastic layer between two sheets of non-adhesive separator film (sold under the trade name CONTIFLEX CONFORMAL COATING). The thermoplastic layer flows into the slots during compression molding to prevent other layers which lie above it from sagging into the slot.
German Patent DE 31 19 884 is directed to a method for manufacturing rigid-flexible printed circuit boards. This method employs a bonding foil arranged on a flexible layer, and a sheet-metal plate coated, e.g., by spraying, with liquid polytetrafluoroethylene. The contour of the sheet-metal plate corresponds to the form of the planned flexible regions. The flexible layer is compression molded with the rigid layer in the area of the bonding foil to form a composite. Printed circuits are formed on the rigid layer. Slots are provided in the composite. A section of the rigid layer may be removed together with the sheet-metal plate.
German Patent DE 26 57 212 relates to a method for producing composite rigid-flexible printed circuit boards having two or more insulating individual layers. The printed circuit boards have adjacent rigid and flexible areas made by pressing together separate rigid and flexible insulating layers with the aid of bonding sheets. Portions of the rigid layer may be removed to provide the desired flexible areas. This method discloses flexible layers that cover the entire printed circuit board.
German Patent DE 26 57 212 discloses a rigid-flexible printed circuit board having slots introduced on a side turned toward a flexible layer before compression molding. The slots define flexible and rigid regions of the printed circuit board. The depth of the slots may correspond to approximately one-half the thickness of the rigid material. After compression molding and formation of printed circuits on the external surfaces, additional slots may be produced in a similar manner but on the opposite side of the rigid layer. The rigid layer occupying the desired flexible regions can be removed along the separating lines of the slots.
The German Patent DE 26 57 212 discloses rigid-flexible printed circuit boards that use adhesive layers which flow during compression molding. Since the material within the slots must eventually be removed, isolating foils are used to prevent the adhesive agent from flowing onto the material which is located in the flexible region. The plastic films used for the flexible layers in these printed circuit boards may be hygroscopic, so that when the circuit arrangement is soldered, delaminations can occur between the flexible and the rigid layers due to the vaporization of absorbed moisture.
Generally, it is also possible to use a non-flowing adhesive agent between the rigid and flexible single layers of rigid-flexible printed circuit boards. The viscosity of the non-flowing adhesive agent is such that it does not flow into the flexible areas during compression molding. Thus, the areas of bonding by the non-flowing adhesive can be controlled. These types of adhesive agents, however, require the application of a known pressure-equalizing cushion during compression molding in order to attain a homogeneous bonding of the flexible and rigid single layers. When circuit-board conductors are also present on a rigid single layer that is located in the interior of a multilayer printed circuit board, this cushion may cause the printed circuit to press into the flexible single layer. The result is a relief formation on the flexible single layer which in turn causes a relief formation on the copper foil applied to the outer flexible layer. Thus, the manufacture of circuit-board conductors on this copper foil using etching techniques is more difficult